My tutorial on look-ahead and look-behind covers this.
You're specifying that the open tag is not followed immediately by another open tag; you want to specify that it's followed by any arbitrary text that isn't an open tag. This s/// expression should do what you want:

Update: I should have broken this down in commented style (I also note that I could have done lookahead for everything after the 3):

$x =~ s{(?<=<td>) # Start match with open-tag
( # Capture
(?:(?!<td>).)* # Any number of characters that do not star+t an open-tag
)3 # Close capture; match literal 3
(?= # Look ahead to match
(?:(?!</td>).)* # Any number of characters that do not star+t a close-tag
</td> # then a close-tag
)} # End lookahead and pattern
{$1a}x;